1. Field of the Invention
The present invention relates to methods for making inks that upconvert infrared light to visible light and that can be printed using inkjet printers.
2. Description of Related Art
Inks and toners that fluoresce when illuminated with ultraviolet light are well known and have been used for many years. These ultraviolet inks are downconverting because they emit light having a wavelength that is greater than the wavelength of the ultraviolet light. That is, the frequency of the emitted light is less than the frequency of the ultraviolet light. For example, an ultraviolet ink may fluoresce at a wavelength of about 500 nanometers (nm, corresponding to a frequency of 600 terahertz (THz), which is in the visible light spectrum) when illuminated by light having a wavelength of about 200 nm (1,500 THz, which is in the ultraviolet spectrum).
Downconverting/ultraviolet inks are used in document security. For example, a security image may be printed onto a document, such as a check, using ultraviolet ink. Because ultraviolet inks are colorless when illuminated by visible light, a counterfeiter may not observe the image and, thus, produce a counterfeit document that does not have the security image in ultraviolet ink. Because ultraviolet inks have been used in document security for years, counterfeiters are now aware of their existence and will illuminate documents using ultraviolet light to determine if an ultraviolet security image is part of the document.
Once a counterfeiter is aware of an ultraviolet security image, replication of the ultraviolet image is relatively easy. Ultraviolet inks are readily available. Ultraviolet inks for use with inkjet printers are also available. Printing using ultraviolet ink on a home use printer is now a relatively simple task.
In order to provide security features that include fluorescing, the inventors have been experimenting with upconverting inks and toners. As opposed to downconverting inks, upconverting inks convert light in the infrared spectrum to light in the visible spectrum. For example, these upconverting inks may fluoresce at a wavelength of about 500 nm (600 THz, which is in the visible light spectrum) when illuminated by light having a wavelength of about 900 nm (331 THz, which is in the infrared spectrum).
Upconverting infrared-responsive inks provide advantages over ultraviolet inks. Infrared-responsive security features are not widely used. The large infrared light source currently required to illuminate a significant portion of an upconverting image makes their use undesirable.
Attempts at making aqueous based upconverting inks have resulted in inks that cannot be used in an inkjet printer. For example, Kellar et al. (U.S. Pat. Pub. No. 2014/0261031) forms an ink by synthesizing and doping upconverting nanocrystals. Kellar et al. then disperses the upconverting nanocrystals into a solvent system. However, they were not able to disperse the upconverting nanocrystal particles into a water based system. The challenge of maintaining upconverting nanoparticles suspended or dispersed in a water based system remains. The ink of Kellar et al. is also expensive due to the cost of the raw material and the processes for synthesizing the upconverting nanocrystals.
Accordingly, there is a need for a relatively inexpensive and stable aqueous-based upconverting ink that is usable in inkjet printers.